Pterin-4 alpha-Carbinolamine Dehydratase (PCD) Deficiency via the PCBD1 Gene

Hyperphenylalaninemias due to tetrahydrobiopterin (BH4) deficiency are a result of a disruption in phenylalanine homeostasis and dopamine and serotonin biosynthesis. These disorders are caused by pathogenic variants in the genes encoding enzymes involved in the biosynthesis or regeneration of BH4. The phenylalanine, tyrosine, and tryptophan hydroxylases all require BH4 as a cofactor, and lack of this cofactor results in secondary hyperphenylalaninemia and, in some disorders, depletion of the neurotransmitters dopamine and serotonin (Blau et al. 2014).

Patients with pterin-4 α-carbinolamine dehydratase (PCD) deficiency, also known as primapterinuria, may be detected via newborn screening due to hyperphenylalaninemia (HPA). In addition to HPA, these patients are also found to have markedly high levels of neopterin in urine, low levels of biopterin, and significant levels of 7-substituted biopterin compounds, mainly primapterin, while CSF pterin and neurotransmitter levels are normal. The increased levels of neopterin and decreased biopterin have resulted in patients being initially misdiagnosed with 6-Pyruvoyl-Tetrahydropterin Synthase Deficiency. Clinically, some patients have presented with neurologic symptoms, such as progressive hypotonia, EEG abnormalities, and delays in motor development (Blau et al. 2014). For reasons that are not understood, hyperphenylalaninemia is transient in PCD deficient patients. Phenylanine levels may rise as high as those observed in classic phenylketonuria patients, but such spikes seem to be temporary and are only observed during infancy. To date, all reported patients have shown normal levels of phenylanine and normal development by early childhood (Citron et al. 1993; Thöny et al. 1998a; Thöny et al. 1998b; Blau et al. 2014).

While PCD deficiency appears to be a primarily transient disorder that can be treated with a low phenylalanine diet in infancy, a few recent reports have suggested that PCD deficiency may lead to development of disease similar to maturity onset diabetes of the young (MODY). This is thought to be due to the fact that PCD has been proposed to be a bifunctional enzyme which also interacts with the HNF1B transcription factor (Blau et al. 2014; Ferrè et al. 2014; Simaite et al. 2014). Defects in the HNF1B gene have been shown to lead to renal cysts and diabetes syndrome (RCAD, also referred to as MODY5). Recommendations in current literature suggest that PCD deficient patients should therefore be monitored for previously unrecognized later onset complications (Ferrè et al. 2014; Simaite et al. 2014).

Pterin-4 α-Carbinolamine Dehydratase deficiency is thought to be inherited in an autosomal recessive manner. The PCBD1 gene (chromosome 10q22, 4 exons) encodes the PCD enzyme. To date, roughly half of the reported pathogenic variants in the PCBD1 are missense, while the remainder are nonsense and small deletions (Human Gene Mutation Database). Pathogenic variants have been reported in exons 2 through 4, although the majority reside in exon 4. While only a small number of patients have been reported in the literature, over 30% of have been homozygous or compound heterozygous for the variant Gln97*, making this the most commonly reported causative variant in the PCBD1 gene to date.

Other inborn errors of BH4 metabolism can present with similar biochemical features and involve 6-pyruvoyl tetrahydropterin synthase (PTS gene), GTP cyclohydrolase I (GCH1 gene), and Dihydropteridine Reductase (QDPR gene) (Blau et al. 2014, Trujillano et al. 2014).

The PCD enzyme is involved in the regeneration of tetrahydrobiopterin (Blau et al. 2014). During the hydroxylation of phenylalanine by the phenylalanine hydroxylase enzyme, the tetrahydrobiopterin (BH4) cofactor is converted to 4α-carbinolaminetetryhydrobiopterin. This compound is subsequently converted to quinoid dihydrobiopterin via the activity of the PCD enzyme. Finally, the Dihydropteridine Reductase (DHPR) enzyme converts the q-dihydrobiopterin back to tetrahydrobiopterin (Citron et al. 1993; Blau et al. 2014). In addition, the PCD enzyme is also thought to act as a dimerization cofactor (and is thus sometimes called DCoH) for the transcription factor HNF-1α, and is thought to stimulate activity of this transcription factor (Blau et al. 2014; Ferrè et al. 2014; Simaite et al. 2014).

Clinical sensitivity is difficult to estimate because only a small number of patients have been reported. Analytical sensitivity should be high because all of the variants reported to date are detectable by direct sequencing.

This test provides full coverage of all coding exons of the PCBD1 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).